Aluminum plating process

ABSTRACT

ALUMINUM PLATING ACCORDING TO WHICH ALUMINUM FILM FORMED HAS NO CRACKS AND PIN-HOLES, A UNIFORM THICKNESS AND EXCELLENT SURFACE LUSTER CAN BE ATTAINED BY CONTACTING A SUBSTRATE COVERED WITH ALUMINUM WITH A SMALL AMOUNT OF A SURFACE TREATING AGENT SELECTED FROM ACTIVE HYDROGENCONTAINING COMPOUNDS, OXYGEN AND HALOGENS AND THEREAFTER TAKING OUT THUS TREATED SUBSTRATE INTO THE AIR.

United States US. Cl. 117-62 18 Claims ABSTRACT OF THE DISCLOSURE Aluminum plating according to which aluminum film formed has no cracks and pin-holes, a uniform thickness and excellent surface luster can be attained by contacting a substrate covered with aluminum with a small amount of a surface treating agent selected from active hydrogencontaining compounds, oxygen and halogens and thereafter taking out thus treated substrate into the air.

This invention relates to an aluminum plating process and more particularly it concerns a process for forming an excellent aluminum film on a substrate by contacting a heated substrate with an alkyl aluminum compound to cause thermal decomposition of said alkyl aluminum compound.

The term a substrate covered with aluminum means a substrate which has been covered with aluminum produced by thermal decomposition of an alkyl aluminum compound caused by contact of a heated substrate with alkyl aluminum compound, but which has not yet been taken out into the air.

K. Ziegler et al. invented a method for plating a substrate with aluminum by contacting a heated substrate with a liquid alkyl aluminum compound or with vapor of an alkyl aluminum compound to cause thermal decomposition of said alkyl aluminum. In this respect, see, for example, Japanese Pat. 234,069. However, such method for plating a substrate with aluminum results in serious drawbacks mentioned below.

That is, there are cracks and pin-holes on the aluminum film formed, which cause deterioration of corrosion resistance, oxidation resistance at a high temperature and electric characteristics of the substrate. Furthermore, the thickness of the aluminum film is not uniform and luster of the surface is extremely poor.

As the results of the inventors research in an attempt to overcome these drawbacks, it has been found that these drawbacks are brought about when a substrate covered with aluminum is taken out into the air. conventionally, no specific treatment had been made to the substrate covered with aluminum when it is taken out into the air.

The inventors have found a method where said drawbacks of the conventional aluminum film are eliminated.

That is, the object of this invention is to provide a method for plating a substrate with aluminum to obtain an aluminum film having no cracks and pinholes, a uniform thickness and an excellent surface luster by contacting the substrate covered with aluminum with a small amount of a surface treating agent selected from active hydrogen-containing compounds, oxygen and halogens and thereafter taking out thus treated substrate into the Other objects will be apparent from the following descriptions.

Alkyl aluminum compounds used in this invention include any alkyl aluminum compounds capable of depositing aluminum by thermal decomposition. Among them, the following are preferable because they are easily thermally decomposed and are economical. That is, di-

atent O alkyl aluminum hydrides, trialkyl aluminums having alkyl groups containing 2-20 carbon atoms such as triethyl aluminum, diethyl aluminum hydride, trinormalpropyl aluminum, triisopropyl aluminum, trinormalbutyl aluminum, dinormalbutyl aluminum hydride, triisobutyl alumium, diisobutyl aluminum hydride, trinormalbenzyl aluminum, trinormalhexyl aluminum, trinormaloctyl aluminum, tri-2-ethylhexyl aluminum hydride and tridecyl aluminum, or mixtures thereof.

Furthermore, the alkyl aluminum compound may be used together with compounds such as alkali metal compounds, ethers, tertiary amines, quaternary ammonium salts, etc. which are capable of producing a complex compound with the alkyl aluminum compound and which are mentioned in U.S. Pats. 3,154,407 and 3,273,996.

The joint use of these compounds results in a high purity aluminum plating.

Furthermore, the alkyl aluminum compound may be used in admixture with inert organic solvents, such as hexane, heptane, octane, cyclopentane, cyclohexane, benzene, toluene, xylene, petroleum, paraffin, alkyl benzene, diphenyl, etc.

Substrates to be plated are, for example, metals such as iron, steel, aluminum, copper brass, pottery, glass, or-

ganic and inorganic resin, etc. The substrate is preferably cleaned prior to formation of aluminum film. The substrate is heated to a temperature higher than the thermal decomposition temperature of alkyl aluminum compound, preferably of 300-600 C. and is contacted with a plating solution or plating vapor. The substrate may be heated by known methods and resistance heating, induction heating, etc. may be employed depending upon kind and shape of the substrate. The induction heating is preferable for continuous heating of especially thin metal sheet.

In order to produce a substrate covered with aluminum in accordance with this invention, a method which comprises contacting a heated substrate with a liquid alkyl aluminum compound or a method which comprises contacting a heated substrate with vapor of alkyl aluminum compound may be used. Election of one of these methods is determined depending upon 'kind and shape of the substrate to be plated. The substrate may be covered with aluminum by thermal decomposition of an alkyl aluminum compound on a substrate by one time heating or by intermittent thermal decomposition due to two or more beatings. The latter is especially preferable. The thermal decomposition may be carried out in the presence of a compound capable of accelerating the thermal decomposition such as titanium chloride, titanium bromide, vanadium chloride, iron chloride, copper chloride, etc. which are mentioned in US. Pat. 3,306,732. Addition of such compounds is useful especially for plating a substrate of low heat stability. The thermal decomposition is required to be carried out in an inert atmosphere, but no critical limitation is present in pressure for operation.

Examples of the surface treating agent to be contacted with substrate covered with aluminum in this invention are oxygen, halogens such as chlorine, fluorine and bromine and active hydrogen-containing compounds having at least one substitutable hydrogen such as water; ammonia; primary or secondary amine compounds such as dimethyl amine, monobutyl amine, etc.; sulfides such as hydrogen sulfide, ethyl thioalcohol, dodecyl thioalcohol, etc.; monoor polyhydric alcohols such as methanol, ethanol, isopropyl-alcohol, butanol, ethylene glycol, propylene glycol, glycerine, etc.; carboxylic acids such as acetic acid, naphthenic acid, stearic acid, adipic acid, maleic acid, phthalic acid, etc.; and inorganic acids such as hydrogen chloride, hydrogen fluoride, hydrogen bromide, nitric acid, etc.

Said active hydrogen-containing compounds are used as a solution which contains 1'010,000 p.p.m., preferably IUD-1,000 p.p.m. (weight basis) of said active hydrogencontaining compound dissolved in aromatic hydrocarbons such as benzene, toluene, xylene, naphthalene, alkylbenzenes, etc., aliphatic hydrocarbons such as pentane, hexane, octane, decene, etc. and mixtures thereof or as an atmosphere of the active hydrogen-containing compound of 0.01-20 mm. Hg, preferably 01-10 mm. Hg. In case of the latter method, the atmosphere is usually diluted with an inert gas and the method is operated under normal pressure or higher pressure. When the amount of the active hydrogen-containing compound is less than or more than the above range, the surface of aluminum film is not effectively improved.

Oxygen is used as a solution which contains 10,000 p.p.m. (weight basis), preferably IOU-1,000 ppm. of oxygen dissolved in aromatic hydrocarbons such as benzene, toluene, xylene, naphthalene, alkyl benzenes, etc., aliphatic hydrocarbons such as pentane, hexane, octane, decene, etc. and mixtures thereof or as an atmosphere containing oxygen of 001-100 mm. Hg, preferably 0.1- 80 mm. Hg.

Halogens are used as an atmosphere of halogen gas of 001-20 mm. Hg, preferably 01-10 mm. Hg or as a solution containing l02,000 p.p.m (weight basis) dissolved in a suitable solvent such as paraffins.

When the surface treating agent is contacted in vapor phase, it is preferable to contact the substrate covered with aluminum with the surface treating agent diluted with an inert gas under normal pressure or higher pressure, instead of using the gas of the surface active agent as it is.

-When the amount of oxygen or halogen is less than or more than the range mentioned above, the surface of aluminum film is not effectively improved.

ISaid treatment is generally carried out at 0-300 (3., preferably of 200 C.

As mentioned above, the method of this invention provides the following advantages as compared with the conventional method according to which a substrate covered with aluminum is taken out into the air without treatments of this method. That is, since oxidation which occurs when the substrate covered with aluminum is taken out into the air without treatments of this method can be prevented, aluminum film formed on the substrate has no cracks and pin-holes on its surface, has a uniform thickness and exhibits excellent surface luster.

The substrate plated with aluminum in accordance with this invention has high corrosion resistance and oxidation resistance at a high temperature and excellent electric characteristics. The substrate can be subjected to metal surface treatment, such as sealing treatment, stabilization treatment and alumite finishing. Because of these excellent characteristics of the aluminum film, the industrial value is extremely great.

The following examples are given by way of specifically illustrating this invention and are not intended to be construed as limiting in any case.

EXAMPLE 1 A steel sheet of 50 mm. x 50 mm. x 0.6 mm. was dipped in a 0.5 weight percent aqueous hydrofluoric acid solution at room temperature for 15 seconds, then was washed with water and subsequently with alcohol and then dried. Thus treated steel sheet was used as a specimen.

The following whole process steps from the heat treatment of the specimen until taking it out into the air were carried out in argon atmosphere.

Said specimen was heated to 400 C. in argon atmosphere and thereafter was dipped in 500 cc. of an alkyl aluminum solution comprising 81% by weight of diisobutyl aluminum hydride, 11% by weight of diethyl aluminum hydride, 5% by weight of triisobutyl. aluminum and 3% by weight of triethyl aluminum at C. After lapse of one minute, the specimen was withdrawn from the solution and maintained in argon atmosphere at C. for 10 minutes to wash away the deposited alkyl aluminum. Said treatment was repeated further 3 times to form aluminum film on the steel sheet.

Then, said specimen was contacted with 500 cc. of hexane containing 0.01% by weight of water at 20 C. for one minute and then taken out into the air. Thus obtained steel sheet plated with aluminum had an excellent luster of silver white color and good surface properties. Thus obtained steel sheet was surrounded by solid paraflin in a width of 10 mm. around the sheet and dipped in 500 cc. of 25 weight percent aqueous nitric acid solution at 20 C. to carry out acid resistance test.

As the results, even after lapse of 40 minutes, no bubbles were generated and no change was observed in the surface of aluminum film. The average thickness of the film was 1.9 determined by increase in the amount.

Instead of water treatment in said method, the specimens were treated with hexane containing 0.01% by weight of ethanol, hexane containing 0.02% by weight of acetic acid, hexane containing 0.04% by weight of monobutyl amine, and hexane containing 0.08% by weight of dodecyl thioalcohol, respectively and were taken out into the air. Each steel sheet having aluminum film had excellent luster of silver white color and good surface properties.

Acid resistance test was also efiected to these specimens to observe no generation of bubbles even after lapse of 40 minutes and no change in the surface of the film.

For comparison, the specimen without said water treatment was cooled in argon and taken out into the air. Thus obtained steel sheet plated with aluminum had a luster of silver White color. This steel sheet was subjected to the same acid resistance test and bubbles were generated after lapse of 30 minutes.

EXAMPLE 2 The same specimen as in Example 1, which was washed in the same manner as in Example 1 was used used in this example. The specimen was pre-heated to 500 C. and was dipped in an alkyl aluminum solution having the same compositions as in Example 1 heated to 200 C. The treatment was carried out in argon atmosphere as in Example 1. After lapse of one minute, the specimen was withdrawn and washed with 500- cc. of hexane. Then, the specimen was contacted with 500 cc. of hexane containing 0.01% by weight of water at 20 C. for one minute and thereafter taken out into the air. The thus obtained steel sheet plated with aluminum had a luster of silver white color and good surface properties. The average thickness of the aluminum film was 2.1 determined by the amount increased.

Said steel sheet was also subjected to the same acid resistance test as in Example 1 and no bubbles were generated even after lapse of 10 minutes.

For comparison, the specimen without said water treatment was cooled in argon and taeken out into the air. The thus obtained steel sheet plated with aluminum had grey color and no luster. This was subjected to the same' acid resistance test as mentioned above and bubbles were generated after lapse of 5 minutes.

EXAMPLE 3 A steel sheet of 50 mm. x 50 mm. x 0.6 mm. was dipped in a 0.5 weight percent aqueous hydrofluoric acid solution at room temperature for 15 seconds, then was Washed with water and subsequently with alcohol and then dried. Thus treated sheet was used as a specimen.

The following whole steps from oxygen treatment of the specimen until taking it out into the air were carried out in argon atmosphere. Said specimen was heated to 400 C. in argon atmosphere and then was dipped in 500 cc. of an alkyl aluminum solution comprising 81% by weight of diisobutyl aluminum hydride, 11% by weight of diethyl aluminum hydride, 5% by weight of triisobutyl aluminum and 3% by weight of triethyl aluminum at 20 C. After lapse of one minute, the specimen was withdrawn and kept in argon atmosphere at 100 C. for 10 minutes to wash away the deposited alkyl aluminum. This treatment was repeated further three times to form aluminum film on the steel sheet. Then, said specimen was contacted with 500 cc. of hexane containing 0.01% by weight of oxygen at 20 C. for one minute and thereafter taken out into the air. Thus obtained steel sheet plated with aluminum had an excellent luster of silver white color and good surface properties. The steel sheet was surrounded by solid parafiin in a width of 10 mm. around the sheet and dipped in 500 cc. of 25 weight percent aqueous nitric acid solution at 20 C. to carry out an acid resistance test. As the result, even after lapse of 40 1nin utes, no bubbles were generated and no change in the aluminum film surface was observed. The average thickness of the film was 1.9 determined by increase in the weight.

For comparison, the specimen without said oxygen treatment was cooled in argon and taken out into the air. The thus obtained steel sheet plated with aluminum had a luster of silver white color. This steel sheet was subjected to the same acid resistance test as mentioned above and bubbles began to generate after lapse of 30 minutes.

EXAMPLE 4 The same specimen as in Example 3, which was washed in the same manner as in Example 3 was used in this example. This specimen was pre-heated to 500 C. and dipped in an alkyl aluminum solution having the same compositions as that of Example 3, which was heated to 200 C.

The treatment was carried out in argon atmosphere as in Example 3. After lapse of one minute, the specimen was withdrawn from the alkyl aluminum solution and washed with 500 cc. of hexane. Then, said specimen was contacted with argon atmosphere containing chlorine gas of 1 mm. Hg for one minute and thereafter, taken out into the air. The thus obtained steel sheet plated with aluminum had a luster of silver white color and good surface properties. The average thickness of the aluminum film was 2.1 1 determined by increase in the weight. This steel sheet was subjected to the same acid resistance test as in Example 3 and no bubbles were generated even after lapse of 10 minutes.

For comparison, the specimen without chlorine gas treatment was cooled in argon atmosphere and taken out into the air. Thus obtained steel sheet plated with aluminum had grey color and had no luster. This steel sheet was subjected to the same said resistance test as mentioned above and after lapse of 5 minutes, bubbles were generated.

From the foregoing results, it will be noted that with the steel sheets plated with aluminum in the same thickness, the luster and properties of the surface treated according to this invention were superior to those of the surface without treatment of this invention.

What is claimed is:

1. In the method for aluminum plating a substrate by contacting a heated substrate in an inert atmosphere with an alkyl aluminum compound to cause thermal decomposition of said alkyl aluminum compound, and wherein the substrate covered with aluminum is thereafter exposed to the air, the improvement which comprises contacting the substrate covered with aluminum produced by the thermal decomposition of said alkyl aluminum compound and before exposure to air but after the thermal decornposition process, with a surface treating agent selected from the group consisting of a solution containing 10,000 p.p.m. of an active hydrogen-containing compound, a gas atmosphere containing the active hydrogen-containing compound of 001- mm. Hg, a solution containing 1010,000 p.p.m. of oxygen, a gas atmosphere containing oxygen of 001-100 mm. Hg, a solution containing 10- 2,000 p.p.m. of a halogen and a gas atmosphere containing a halogen of 001-20 mm. Hg, at a temperature from 0 to 300 C.

2. A method according to claim 1, wherein said alkyl aluminum compound is selected from dialkyl aluminum hydride, trialkyl aluminum having alkyl groups containing 220 carbon atoms and mixtures thereof.

3. A method according to claim 1, wherein the substrate heated to 300600 C. is contacted with liquid or vapor of the alkyl aluminum compound to plate the substrate with aluminum.

4. A method according to claim 1, wherein the active hydrogen-containing compound is selected from the group consisting of Water, ammonia, primary and secondary amine compounds, sulfides, monoand poly-hydric alcohols, carboxylic acids, and inorganic acids.

5. A method according to claim 4, wherein the primary and secondary amine compounds are selected from the group consisting of dimethyl amine, and monobutyl amine.

6. A method according to claim 4, wherein the sulfide is selected from the group consisting of hydrogen sulfide, ethyl thioalcohol, dodecy thioalcohol.

7. A method according to claim 4, wherein the monoand poly-hydric alcohols are methanol ethanol, isopropylalcohol, butanol, ethylene glycol, propylene glycol and glycerine.

8. A method according to claim 4, wherein the carboxylic acid is selected from the group consisting of acetic acid, naphthenic acid, stearic acid, adipic acid, maleic acid and phthalic acid.

9. A method according to claim 4, wherein the inorganic acid is selected from the group consisting of hydrogen chloride, hydrogen fluoride, hydrogen bromide and nitric acid.

10. A method according to claim 1, wherein the halogen is selected from chlorine, fluorine and bromine.

11. A method according to claim 1, wherein the surface treating agent is oxygen dissolved in an aromatic hydrocarbon, an aliphatic hydrocarbon or mixtures thereof.

12. A method according to claim 1, wherein the surface treating agent is oxygen diluted with an inert gas.

13. A method according to claim 1, wherein the surface treating agent is a halogen dissolved in a solvent comprising paraffins.

14. A method according to claim 1, wherein the surface agent is halogen diluted With an inert gas.

15. A method according to claim 1, wherein the surface treating agent is an active hydrogen-containing compound dissolved in an aromatic hydrocarbon, aliphatic hydrocarbon or a mixture thereof.

16. A method according to claim 1, wherein the surface treating agent is an active hydrogen-containing compound diluted with an inert gas.

17. A method according to claim 1, wherein the substrate covered with aluminum is contacted with the surface treating agent at a temperature of from 15200 C.

18. An aluminum plated substrate produced according to the method of claim 1.

References Cited UNITED STATES PATENTS 2,990,295 6/1961 Breining et al. 117107.2 R X 3,041,197 6/1962 Berger 117-130RX 2,867,546 1/195'9 MacNevin 117107.2 R X 3,462,288 8/1969 Schmidt et al. l17-107.2 R X 2,798,051 7/1957 Bicek 117107.2 R X 3,086,881 4/1963 Jenkin 117--130 R X 3,155,532 11/1964 Basile 117-130 R X ALFRED L. LEAVITT, Primary Examiner J. R. BATTEN, JR., Assistant Examiner US. Cl. X.R.

ll7107.2 R, R, R; l486.3, 6.27 

